1. Field of the Invention
The present invention relates to a stepping motor, and more specifically to a stepping motor with its axial dimension reduced.
2. Description of the Related Art
Recently, stepping motors, which function to readily perform positional or angular control according to digital signals, are used widely in various equipments, such as a printer, a facsimile machine, a floppy disk drive (registered trademark), and the like.
For example, Japanese Patent Application Laid-Open No. H08-182298 discloses a claw-pole type PM (permanent magnet) stepping motor, usually called simply as PM stepping motor, which comprises a rotor assembly composed of a ring magnet circumferentially magnetized, and a stator assembly composed of two stator units axially stacked (refer to FIG. 1 therein). Since the PM stepping motor (hereinafter referred to simply as stepping motor) has two stator units stacked axially, reduction in its axial dimension is inhibited. In order to overcome the problem, Japanese Patent Application Laid-Open No. H10-94237 and PCT International Application (Japanese Translation) No. 2003-500998 disclose stepping motors structured to have a stator assembly composed of two segments (stator units) which are shaped semi-annular and which are arranged horizontally on a same plane so as to be symmetric about a rotary shaft of a rotor. The semi-annular stator units each include a cylindrical coil which has its axis oriented parallel to the rotary shaft, and has its axis positioned outside the rotor.
The present inventors have proposed stepping motors with a lowered profile in Japanese Patent Applications Nos. 2002-283511, 2002-303904 and 2002-345380 (these applications have not been published yet and therefore do not constitute prior arts).
FIG. 1 attached herein is an exploded perspective view of one of the stepping motors proposed in the aforementioned Japanese Patent Application No. 2002-283511. The stepping motor shown in FIG. 1 comprises: a stator assembly 30 composed of two segments, namely, a first stator unit 30A and a second stator unit 30B which are arranged horizontally on a same plane; and a rotor assembly 50 rotably disposed inside the stator assembly 30.
The first stator unit 30A comprises first upper and lower stator yokes 31a, 34a, a cylindrical spacer 44a1 formed of a magnetic material, and a cylindrical coil unit 40A including a coil 42a for exciting. The first upper and lower stator yokes 31a, 34a have respective pole teeth 32a1 to 32an and 35a1 to 35an. 
The second stator unit 30B comprises second upper and lower stator yokes 31b, 34b, a cylindrical spacer 44b1 formed of a magnetic material, and a cylindrical coil unit 40B including a coil 42b for exiting. The second upper and lower stator yokes 31b, 34b have respective pole teeth 32b1 to 32bn and 35b1 to 35bn. 
The first and second upper stator yokes 31a, 31b are connected to each other at two center line portions having respective slit cuts 37 adjacently aligned therewith, and the first and second lower stator yokes 34a, 34b are connected to each other at two center line portions having respective slit cuts 38 adjacently aligned therewith.
The first stator unit 30A has its upper and lower pole teeth shifted from each other by 180 degrees in terms of electrical angle, also the second stator unit 30B has its upper and lower pole teeth shifted from each other by 180 degrees in terms of electrical angle, and two pole teeth positioned adjacent to each other via the slit cuts 37 or 38 are shifted from each other by either 90 or 270 degrees in terms of electrical angle.
The rotor assembly 50 comprises a rotary shaft 55, a rotor sleeve 52 fixedly attached onto the rotary shaft 55, and a ring magnet 51 fixedly attached onto the rotor sleeve 52 and having a plurality of magnetic poles circumferentially arranged alternately with N and S poles at a regular interval.
In the stepping motor disclosed in the aforementioned Japanese Patent Application Laid-Open No. H10-94237, since distances from the coil to the pole teeth vary greatly from one pole tooth to another, magnetic flux generated by the coil is not evenly carried through to the pole teeth, which causes variation in magnetic flux density and therefore also in torque generation from one pole tooth to another. Specifically, magnetic flux density at pole teeth positioned closest to the slit cuts 37, 38, in other words, farthest from the coil is the lowest, and therefore torque generated at these pole teeth is smaller than torque generated at other pole teeth that are positioned closer to the coil and consequently have a higher magnetic flux density, thus causing variation in torque.
The stepping motor disclosed in the aforementioned PCT International Application (Japanese Translation) No. 2003-500998 has a structure similar to that of the stepping motor described above, and suffers the aforementioned problem of variation in torque generation from one pole tooth to another. Further, since the two stator units are connected to each other via interconnecting portions having a large area located between respective stator yokes, magnetic fluxes generated by respective coils are allowed to easily interfere with each other, which deteriorates torque generation, especially, at pole teeth positioned near the interconnecting portions, and which results in increasing variation in torque generation.
Also, as for the stepping motors proposed in the aforementioned Japanese Patent Applications Nos. 2002-283511, 2002-303904 and 2002-345380, the structure is substantially similar to that of the stepping motors described above, and therefore the same problem is involved. The problem is demonstrated in FIG. 2 showing a graph of magnetic flux distribution at each pole tooth, achieved when current is caused to flow in the coils 42a, 42b of the stepping motor shown in FIG. 1. As shown in FIG. 2, magnetic flux density varies from one pole tooth to another such that the density is the highest at pole teeth positioned closest to the coils (represented by 0, 180 and 360 degrees), then decreases gradually at pole teeth positioned sequentially farther therefrom, and becomes the lowest at pole teeth positioned farthest therefrom, that is to say, closest to the interconnecting portions between the first and second stator yokes. The variation in magnetic flux density leads to variation in torque generation.
Further, in the aforementioned stepping motors, the spacer, which is formed of a magnetic material, disposed between the upper and lower stator yokes, and which constitutes a part of the magnetic circuit of the stator yokes, is fabricated usually by machining therefore prohibiting cost reduction. To overcome this problem, in the stepping motor proposed in the aforementioned Japanese Patent Application No. 2002-303904, the spacer is structured such that a part of each upper and lower stator yoke is drawn so as to form a protrusion adapted to perform spacing function, whereby the number of components is reduced. However, since the spacer thus drawn of the stator yoke plate has its thickness diminished due to the drawing process, and also since the magnetic characteristic of the stator yoke material is deteriorated due to the stress caused by the drawing process, sufficient magnetic force is not generated.